The federal government of the United States has promulgated test methods in 40 CFR Part 60, Appendix A for determining stack gas velocity, volumetric flow rate and moisture content. If one knows the flow rate and has another monitor which measures the concentration of pollutants in a selected volume of fluid one can calculate the quantity of pollutants emitted over any selected time period. Accordingly, the test methods have been used in various ways, including the verification of the performance of continuous emission monitoring equipment required by other rules.
The United States has additional regulatory requirements in the form of 40 CFR, Parts 72 through 75 (acid rain reduction), which utilize the Appendix A methods. Some recent regulations now require many electric utilities to continuously measure emissions of specified pollutants on a mass per unit time basis. Adoption of these rules has put a new importance on the errors in both the continuous monitor and in the referenced test methods. The new regulations establish monetary value in the form of trading credits to a ton of SO.sub.2 emissions. The value of such emissions is such that for large utilities as much as $1,000,000 per percent error in measured emissions may result.
The methods of Appendix A were introduced into law over 20 years ago. They, in general, use simple laboratory apparatus and manual techniques to make the various measurements. Unfortunately, the methods are error prone and tests under the same conditions often yield different results. There are many sources of error related to the care, speed and experience of the personnel performing the method as well as variability of the test hardware itself. Over the years, the need to reduce the errors in these methods have been the subject of much discussion and little action.
Appendix A of Title 40 of the United States Code of Federal Regulations contains two methods for measuring flow which are used to determine compliance with emission regulations. These methods, known as EPA Methods 1 and 2, have gained prominence because they are used to determine the proper location, as well as to verify the performance of continuous measuring flow monitors. Errors in Method 2 data can be very costly to both the supplier of the monitor and the utility. The supplier is affected because the method can erroneously show the monitor is not meeting the performance guarantee. The utility is affected because the method is used to adjust the continuous monitor. If the method is in error, that error will directly cause an enormous high or low use of the utility's SO.sub.2 allowance and SO.sub.2 trading credits.
Our United States patent application Ser. No. 08/238,262, filed May 4, 1994, discloses an automated probe for measuring velocity and flow rate of stack gases. The probe is made and sold by United Sciences, Inc. under the trademark AUTOPROBE 2000. However, prior to the present invention this device was not able to determine the moisture content of stack gases.
The equation for calculating flow velocity from the delta-pressure of a pitot tube contains the molecular weight of the gas being measured. In performing the required testing methods the technician is therefore required to determine the molecular weight of the stack gases being monitored. This determination includes measuring moisture content of the stack gases. Method 4 of Appendix A of Title 40 of the United States Code of Federal Regulations describes two procedures for determining moisture content in stack gases and goes on to say that alternative means for approximating the moisture content including use of wet bulb-dry bulb temperature techniques are acceptable. Determination of the stack gas moisture concentration of water vapor by the procedures described in Method 4 are cumbersome. Similarly, the wet bulb-dry bulb techniques which have been used in the past are also cumbersome insomuch as their operation was manual and the data interpretation, consisting of identifying the wet-bulb temperature, reading a psychometric chart and performing barometric compensation was also manual.
There is a need for an accurate and convenient method and apparatus for measuring moisture content of stack gases which can be used for emissions monitoring. Moreover, the apparatus must be durable and not adversely affected by temperature changes and other environmental conditions found in power plants and other monitoring sites.